1. Field of the Invention
The present invention relates to a tool gripper of a machine tool, particularly to a method and a tool gripper of a machine tool for exchanging tools set to a spindle and a tool magazine of the machine tool.
2. Description of Related Art
There has been conventionally known a machine tool capable of exchanging tools between a spindle and a tool magazine. A tool gripper is installed to such a machine tool, tools respectively set to a spindle and a tool magazine are grasped by pairs of fingers installed to a main body of the tool gripper, and tools are exchanged by changing positions thereof by pivoting a tool exchange arm.
There has been known a structurally simple finger opening and closing mechanism of this kind disclosed in JP-A-3-92238 and U.S. Pat. No. 4,833,772. A center shaft installed with finger driving cams is separately installed at the inside of a hollow pivoting shaft for pivoting a tool exchange arm that accurately regulates motion of the fingers by which pairs of fingers which are urged by springs are opened or closed.
However, such a tool gripper grasps tools only by spring force and therefore, centrifugal force exerted on a tool exceeds the grasping force of the tool exerted by a spring mechanism depending on a pivoting speed of the tool exchange arm. Thus, the tool may be detached from the pair of fingers installed at the inside of the tool exchange arm.
Further, according to such a tool gripper, a pivoting shaft in a hollow section must be fabricated and therefore, the fabrication cost is increased. Furthermore, in respect of the center shaft for opening and closing claw portions at front ends of a pair of fingers, the center shaft is obliged to have a slender shape since it is incorporated at the inside of the pivoting shaft. Accordingly, strength of the center shaft against torsion is small and it is conceivable that even if a pivoting angle of the center shaft is set, an opening and closing angle of the claw portions at the front ends of the pair of fingers is not provided with a predetermined value and therefore, it is necessary to pivot the center shaft excessively in consideration of the torsion. Also, the life of the center shaft is shortened by the torsion.